1. Field of the Invention
The invention relates to a method for the crucible-free float zoning or floating zone pulling of semiconductor rods. More particularly, it relates to a method in which a feed or supply rod is converted into a crystal rod having a molten zone which overlays the crystallization front like a cap, which cap is produced by means of a circular induction heating coil while the crystal rod is subjected to a rotary movement. An induction heating coil having coil ends which are separated from each other by a coil slot is disclosed for carrying out the method.
2. Description of the Prior Art
The production of dislocation-free monocrystalline semiconductor rods with the aid of the method of crucible-free floating zone pulling or float zoning is well known. In addition, numerous induction heating coils which can be used in such methods are known such as that disclosed in U.S. Pat. No. 4,749,837. In most of these methods, a feed rod, which is, as a rule, polycrystalline and is composed, for example, of high-purity silicon obtained by gas-phase deposition is clamped vertically in a receiving container. The receiving container allows a working atmosphere of a particular composition to be established. The rod is melted starting from its lower end by means of an induction heating coil which concentrically surrounds the molten zone. As an option, the rod may be rotated during this melting procedure.
The product rod is usually also rotated and is, as a rule, monocrystalline. It is covered at its upper end in a cap-like manner by molten material and is situated underneath the coil and grows as a result of the progressive crystallization of the material. Such methods are explained, in principle, for example in W. Dietze, W. Keller and A. Muhlbauer, "Float Zone Grown Silicon" in Crystals, Volume 5 (1981), Springer-Verlag, Berlin-Heidelberg-New York. The known pulling methods reach their useful limit when the production of crystalline rods having large diameters, i.e., 10-15 cm and over is required. Such large diameters are being used with increasing frequency in the production of electronic or power components. It has been found that with such rod diameters, some problems which are negligible in pulling smaller rods become increasingly important.
These problems are frequently associated with the fact that the molten cap overlaying the product rod contains a correspondingly larger amount of molten material which also requires a correspondingly higher power to produce. For example, because of the non- homogeneity of the high-frequency field produced by the conventional coil geometries, irregular melting and crystallization behavior is produced. Thus, melt-backs frequently occur in the region of the coil slot and the rods produced have a tendency to exhibit "egg formation", i.e. acentric growth due to the irregular development of the molten cap. These rods also exhibit radial variations in the distribution of dopant. This undesirable effect becomes perceptible even with smaller rod diameters. The pulling of satisfactory rods, in particular with diameters of 10 cm and over, with satisfactory and economically acceptable yields is therefore not possible with the conventional pulling methods even when they are utilized with great care.